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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>3、派生选择器&父子选择器</title>
    <style>
        strong em{
            color: green;
        }
        strong .content{
            color: red;
        }
    </style>
</head>
<body>
    <strong>
        <em>你好！</em>
    </strong>
    <strong>
        <em class="content">你好！</em>
    </strong>
</body>
</html>